What is the mobo's drain on the CMOS battery?

[BernieDD]

Does anybody know the amount of current supplied by the CMOS battery to the motherboard ? I'm trying to fix a 333 MHz Pentium 2 motherboard where the last 2 batteries lasted only 3 months. (Mobo works fine until the new batteries died.) Both batteries came from the same shop and therefore could possibly have been both bad (same production batch). I hooked up my multitester in series with a 3 volt battery connected temporarily by alligator clips to the mobo and read 2 microamperes drain with the motherboard off. I thought this figure might be reasonable but then I could be wrong. Thanks for any help, guys.

 

[Lord Evermore]

30 µA according to this one.

This one claims nano-amp level draw!

It seems that it depends on the motherboard how much current it normally takes, and repeated draining of the battery may be due to a short caused by the way the board is mounted, or due to the board just being defective.

 

[Zepper]

Yeah, 2uA sound about right with the power plugged in and standby voltage present. Try measuring the current from the battery with the power off (I mean unplugged - IOW, with the PSU switch turned off or totally unplugged). It should be higher as I believe the typical CMOS power circuit should look like this:
. v-CMOS battery
o--Ground -----|i|i|i---->|---o---o to CMOS chip
............................diode ^...D - diode
............................................|
o---+3.3V from system---|
.
. The Li coin battery isn't rechargeable and the +3.3V from the system should keep the battery from participating much in the current supply to the CMOS when the standby or regular 3.3V is present. If you turn your machine totally off much of the time such as with the switch on the PSU, switch on a surge protector or unplug, then the battery may decline faster.
. The "...30 uA... " article must be really old as I haven't seen an external battery connector on a new mobo in years. IAC, the battery you buy WILL have a great impact on longevity. Make sure you buy Li coin batteries made in USA, Japan or Switzerland. I get Duracell made in Japan at Staples for approx. $2. every day. In my experience with watches and computers, those made in China will have a much shorter life. Not too familiar with Taiwanese coin batts. Name brands (Duracell, Energizer, Maxell, Panasonic, etc.) usually have a shelf-life date on the package which provides some reassurance. I try to buy with 2 yrs or more left.
. And be sure you're buying the right battery. The equivalent to the CR-2032 (220 mAh (milliAmp-hrs)) is the one you want, the BR-2032 version has 30 or more mAh LESS capacity and the CR/BR-2016 (which on casual inspection looks about the same size) is less than half the mAh capacity (BR/CR = 75/90) (all ratings here are Panasonic specs). So the CR-2032 could ideally last for about 13 yrs at a your constant 2 uA drain. The Panasonic chart for the CR-2032 says the standard drain is 0.2 mA, which is 100x what you measured. So you must be buying Chinese batteries of the wrong type/rating...
.bh.

 

[Zepper]

I found a data sheet on the stand-alone Dallas Real-time Clock-NVRAM IC and the CMOS battery current was listed as 0.5 mA! You would have to wade thru the southbridge data manual to find the value for your mobo as the RTC/NVRAM is integrated now. Oh yeah, that's a PII mobo so it might still be in a standalone IC. The Motorola RTC was a common one, but I don't see it listed in the current parts list - I didn't look in the archives. The CMOS RAM was in the keyboard controller IC, believe it or not...
.bh.

 

[Geniere]

Two thoughts:

- Replace chip

- use several batteries in parallel or a larger capacty one. (this only extends time between failures)


Regards